It is important to monitor the concentration of hydrogen dissolved in molten metals, and in particular in molten aluminium and its alloys. The solubility of hydrogen in molten aluminium is much higher than its solubility in solid aluminium, and therefore when aluminium is cast there is a tendency for dissolved aluminium in the melt to form bubbles or other flaws in the solid aluminium product. The hydrogen concentration in molten aluminium can rise through reaction of the aluminium with moisture in the environment, and so it is critical to be able to monitor hydrogen concentration during aluminium casting.
Many methods have been developed for monitoring hydrogen concentration in molten aluminium and its alloys, and in other metals, but all of these suffer disadvantages such as lack of accuracy, a requirement for cumbersome apparatus, and disadvantageously long measurement times. A technology which offers solutions to these problems is the possibility of using a proton-conducting solid-electrolyte sensor with an internal solid-state hydrogen reference. This technology has been described in published prior art, including ‘The Detection of Hydrogen in Molten Aluminium’ by D P Lapham et al, Ionics 8 (2002), pages 391 to 401, ‘Determination of Hydrogen in Molten Aluminium and its Alloys using an Electrochemical Sensor’ by C Schwandt et al, EPD Congress 2003, TMS (The Minerals, Metals and Materials Society), 2003, pages 427 to 438, and in International patent application No. PCT/GB2003/003967 of Cambridge University Technical Services Limited. All of these documents are incorporated herein by reference in their entirety. An advantageous method for taking measurements from such a probe, termed the ‘reverse current technique’ has been described in European patent application No. EP 98932375.3 of D J Fray and R V Kumar, which is also incorporated herein by reference in its entirety.
However, this technology has not, to date, been developed to produce a hydrogen probe which meets the practical requirements of shop-floor use in a foundry. The present invention aims to address this problem.